1. Field of the Invention
The present invention relates to an image processing apparatus which is configured to transfer an image formed on an image carrier onto a recording medium, such as a sheet, to thereby form an image on the recording medium, and a control method therefor.
2. Description of the Related Art
Conventionally, the following methods are known which enable an image processing apparatus to maintain stability of image quality. One example is a method in which a gradation pattern or the like specific pattern is formed on a sheet, and then the gradation pattern information read by an image reader is supplied to image forming conditions including γ correction in a feedback manner to thereby enhance the stability of image quality. Further, long-term use of the image processing apparatus can cause a change in the adhesion characteristic of developing toners with respect to the potential on the photosensitive drum potential, which makes it difficult to obtain an optimum image quality only by the γ correction performed by the feedback of gradation pattern information thereto. To overcome this problem, a technique has been disclosed e.g. in Japanese Laid-Open Patent Publication No. 2000-238341, in which the stability of image quality is maintained by adjusting density correction characteristics according to the relationship between gradation pattern information read by the image reader and the densities of images (patches) formed on a photosensitive member in predetermined timing.
However, when the method proposed in Japanese Laid-Open Patent Publication No. 2000-238341 is employed, the γ correction is possible in a preset density range of patch levels, but in the other density ranges, the γ correction sometimes cannot provide sufficient effects. Further, in the conventional image processing apparatus, image density is stabilized mainly with respect to a range of process gray (gray generated by mixing three colors Y (yellow), M (magenta), and C (cyan)). For this reason, colors actually printed on a sheet cannot always have sufficient stability with respect to memory colors (sky blue, pale peach color, etc.) normally imaged by human beings.